Hydration of krypton and consideration of clathrate models of hydrophobic effects from the perspective of quasi-chemical theory

Henry S. Ashbaugh, D. Asthagiri, Lawrence R. Pratt, Susan B. Rempe

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Abstract

Ab initio molecular dynamics (AIMD) results on a krypton-water liquid solution are presented and compared to recent XAFS results for the radial hydration structure for a Kr atom in liquid water solution. Though these AIMD calculations have important limitations of scale, the comparisons with the liquid solution results are satisfactory and significantly different from the radial distributions extracted from the data on the solid Kr/H2O clathrate hydrate phase. The calculations also produce the coordination number distribution that can be examined for metastable coordination structures suggesting possibilities for clathrate-like organization; none are seen in these results. Clathrate pictures of hydrophobic hydration are discussed, as is the quasi-chemical theory that should provide a basis for clathrate pictures. Outer shell contributions are discussed and estimated; they are positive and larger than the positive experimental hydration free energy of Kr(aq), implying that inner shell contributions must be negative and of comparable size. Clathrate-like inner shell hydration structures on a Kr atom solute are obtained for some, but not all, of the coordination number cases observed in the simulation. The structures found have a delicate stability. Inner shell coordination structures extracted from the simulation of the liquid, and then subjected to quantum chemical optimization, always decomposed. Interactions with the outer shell material are decisive in stabilizing coordination structures observed in liquid solution and in clathrate phases. The primitive quasi-chemical estimate that uses a dielectric model for the influence of the outer shell material on the inner shell equilibria gives a contribution to hydration free energy that is positive and larger than the experimental hydration free energy. The 'what are we to tell students' question about hydrophobic hydration, often answered with structural clathrate pictures, is then considered; we propose an alternative answer that is consistent with successful molecular theories of hydrophobic effects and based upon distinctive observable properties of liquid water. Considerations of parsimony, for instance Ockham's razor, then suggest that additional structural hypotheses in response to 'what are we to tell students' are not required at this stage.

Original languageEnglish
Pages (from-to)323-338
Number of pages16
JournalBiophysical Chemistry
Volume105
Issue number2-3
DOIs
StatePublished - Sep 1 2003
Externally publishedYes

Funding

The work was supported by the US Department of Energy, contract W-7405-ENG-36, under the LDRD program at Los Alamos and Sandia, LA-UR-02-6362.

FundersFunder number
US Department of EnergyW-7405-ENG-36, LA-UR-02-6362

    Keywords

    • Ab initio molecular dynamics
    • Clathrate hydrate
    • Hydrophobic hydration
    • Krypton
    • Quasi-chemical theory
    • Scaled particle theory

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